One of the enduring mysteries in physics is high critical-temperature superconductivity—or high-Tc superconductivity. All superconductors (materials that conduct electricity with no resistance) require very low temperatures compared with room temperature, but the high-Tc superconductors have transition temperatures that are higher than their conventional cousins—30° to 110° Celsius above absolute zero, compared to a few degrees. This slight bump in allowable temperatures makes high-Tc superconductors a bit more reachable experimentally, but exactly how they conduct electricity is still mysterious.

A new study published in Science examined a particular class of high-Tc superconductor, known as an iron pnictide. ("Pnictide" refers to an atom in the same column as nitrogen in the periodic table.) K. Hashimoto et al. found evidence of a quantum critical point (QCP): a place where the material's properties change radically due to quantum fluctuations rather than changes in temperature or pressure. While many physicists suspect the presence of a QCP in high-Tc superconductors, none have found unambiguous evidence for its existence. The current study is still not definitive, but the particular iron pnictide material the researchers used provides far cleaner data—and stronger hints that the QCP is actually there. Its presence would reveal a great deal about the inner workings of high-Tc superconductors, perhaps helping lead to even higher temperature superconducting devices.

The phases of matter many of us learned in school—solid, liquid, and gas—are based on the ordering of atoms within materials. Additional phases, including superconductivity, are explicitly quantum in character, relying as they do on the ordering of electric charge carriers. The properties of these charge carriers arise from interactions rather than being fundamental particles like electrons, but they still act like particles: they may have mass, charge, and spin. Materials can change from one phase to another when the temperature or pressure is changed, though the superconducting phase transition can also be induced by introducing atoms whose electrical properties provide extra charge carriers. This process is called doping.

In the case of high-Tc superconductors, the key parameters are temperature and doping. The iron pnictide superconductor in the recent study was BaFe2(As1-xPx)2, where "x" is the doping fraction. (In this case, the pnictide is the arsenic.) The researchers picked this particular pnictide due to the ease with which pure crystals of the material can be grown and how clean the resulting data is. For x values roughly between 0.2 and 0.7, BaFe2(As1-xPx)2 is a superconductor; outside those values, the material isn't superconducting at any temperature.

A QCP—if it is present—marks another type of phase transition, where quantum fluctuations at absolute zero change the superconducting behavior of the material. While absolute zero isn't experimentally achievable, the quantum fluctuations start at (relatively) higher temperatures, changing the behavior of the flow of the charge carriers. One measure of the flow is known as the London penetration depth. (This quantity actually determines how far magnetic fields can penetrate into the superconductor. An ideal superconductor repels all magnetic fields.) Near the hypothetical QCP, the penetration depth grows to large values.

The researchers found that at the optimal doping value, the London penetration depth jumped sharply. While this behavior doesn't automatically mean there is a QCP, it's certainly suggestive and could explain other phenomena in iron-based superconductors. A side effect of a QCP is to divide the superconducting behavior into two regions, based again on doping. In one of these superconducting phases, both superconductivity and magnetism may be able to coexist, a phenomenon not seen in other materials. Hunting for the telltale signs of this second phase transition could be a next step, as the authors stated.

If the QCP is actually present, it may be the driving factor for high-Tc superconductivity. The current study could prove to be significant progress toward solving the enigma of these materials.

... 30° to 110° Celsius above absolute zero, compared to a few degrees

That's a rather poor way of phrasing it. Why involve Celsius? It has the same unit magnitude as Kelvin so 30 to 110 degrees above absolute zero (where Kelvin is implied since you are talking about absolute zero) is a much easier way of expressing this.

so 30 to 110 degrees above absolute zero (where Kelvin is implied since you are talking about absolute zero) is a much easier way of expressing this

Some of the US audience might assume degrees Fahrenheit if they weren't scientists (or lent that way). Though I think expressing it in Kelvin would have been fine. So long as it's not ambiguous it's okay.

Also, it really kinda saddens me that the coolest articles on Ars always have the fewest comments. Superconductivity has the potential to radically alter the world we live in. If this work leads to true "room temperature" superconductive materials, bob's your uncle.

Imagine energy transmission with 0 loss. Or maglev trains that are highly efficient. Or NMR (MRI, whatever) machines that aren't the size of a room.

Edit: Regarding the C vs. K debate, 2 points: there is no such thing as "degrees K", so that would be technically incorrect, and many non-scientists have never even heard of the Kelvin scale, but everyone has heard of the Celsius/centigrade scale.

K. Hashimoto et al. found evidence of a quantum critical point (QCP): a place where the material's properties change radically due to quantum fluctuations rather than changes in temperature or pressure

Urk. It's eminently possible to see pressure-drive quantum critical behavior. It's arguably easier than doing it as a function of doping since pressure is a continuously tunable variable and measuring a range of doping requires a series of different samples.

Regarding the C vs. K debate, 2 points: there is no such thing as "degrees K", so that would be technically incorrect,

touché

TheFerenc wrote:

... and many non-scientists have never even heard of the Kelvin scale, but everyone has heard of the Celsius/centigrade scale.

Hmm, in Canada you can't get out of high school without at least ONE science course introducing the Kelvin system. Maybe it's different in the US since Fahrenheit doesn't convert as nicely as Celsius does, and therefore it's not mentioned in intro courses?

ShuggyCoUk wrote:

Some of the US audience might assume degrees Fahrenheit if they weren't scientists (or lent that way). Though I think expressing it in Kelvin would have been fine.So long as it's not ambiguous it's okay.

Faced with this point, paired with TheFerenc's first point, I have to concede the phrasing in the story doesn't seem as bad.

I suppose the best phrasing in the story would be:

"30 to 110 Kelvin above absolute zero, compared to a few Kelvin"

But considering my own faux pas (referencing "degrees Kelvin") I shouldn't be quite so critical. :-P

Anyone care to attempt a swing about the quantum fluctuations? I took all the qm that I could in school and I don't recall these being discussed in any detail. I imagine these fluctuations are related to the virtual particles attendent with our various forces? From the article it seems like quantum fluctuations are considered the mechanism of change when everything else is otherwise controlled.

I think the P is only silent in English. Where languages are less restrictive, the letters may all be pronounced as written. If I'm not wrong, the word is derived from Greek (as many scientific terms are).

Also, it really kinda saddens me that the coolest articles on Ars always have the fewest comments. Superconductivity has the potential to radically alter the world we live in. If this work leads to true "room temperature" superconductive materials, bob's your uncle.

Call me a skeptic but I have doubts as to the real world usability of any materials that lose their useful properties near room temperature.

Quote:

Imagine energy transmission with 0 loss. Or maglev trains that are highly efficient. Or NMR (MRI, whatever) machines that aren't the size of a room.

Transmission losses are not that high today, thanks to high voltage DC transmission lines. And anyway, transmission lines have to work at higher than room temperature, they need to work at whatever the outside temperature may be. I don't know how hot current transmission lines get over desert regions, but with an acceptable safety margin I'd expect they'd have to work somewhere near 150-200F. And even then I still think the potential damage of even a small part of the line going over the critical temperature would make them a bad idea and a poor risk.

Quote:

Edit: Regarding the C vs. K debate, 2 points: there is no such thing as "degrees K", so that would be technically incorrect, and many non-scientists have never even heard of the Kelvin scale, but everyone has heard of the Celsius/centigrade scale.

I think the P is only silent in English. Where languages are less restrictive, the letters may all be pronounced as written. If I'm not wrong, the word is derived from Greek (as many scientific terms are).

Ah you're probably right. We English speakers also generally leave the "p" off of pneumatic and pneumonia

Call me a skeptic but I have doubts as to the real world usability of any materials that lose their useful properties near room temperature.

I think you'd agree that maglev trains and MRIs are useful. As are magnetic bearings. And particle accelerators.

'room-temperature' superconductors are a lot easier to keep cool than current YBCO based superconductors (liquid nitrogen or loud, vibrating cryocoolers) and especially so compared to type 1 superconductors (liquid helium, dilution refrigeration or even bulkier and expensive cryocoolers).

There are many applications (particularly industrial) that could use superconductors, but are either impractical or costly/niche due to the need for 70 K operations. Even something that is superconducting at, say, -18 C would start to change that. (Quantum computer in your freezer?

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Transmission losses are not that high today

7% adds up.

There is active research into superconducting transmission lines and also electric machines using liquid nitrogen for cooling. The question isn't whether it's technically feasible to operate it, it's more a question of the cost-benefit of running them. I went to a plenary session in 2008 that hypothesized that electric aircraft of the future could benefit from superconductor-based motors due to the weight advantages.

"Additional phases, including superconductivity, are explicitly quantum in character, relying as they do on the ordering of electric charge carriers. The properties of these charge carriers arise from interactions rather than being fundamental particles like electrons, but they still act like particles: they may have mass, charge, and spin."

I love the fact that charge carriers act like particles...

"Also, it really kinda saddens me that the coolest articles on Ars always have the fewest comments. "

Nice, but at Tc = 110 K they are still lagging behind some of the more exotic cuprates around; although thats not the whole story of course, YBCO is often preferable to BSCCO due to its higher Bc despite a lower Tc.

Well thats going back to basic electromagnetism - you get a magnetic field whenever electrons travel through a wire, and vice versa. If you have a material that can conduct electricity infinitely well then any magnetic field will induce a current on the surface of the material and not penetrate beyond the first few 10s of nanometres, which is what causes the Meissner effect.

Essentially 'expending it's strength attempting to induce the strongest current it possibly can?

or is my understanding overly simplistic?

quoting from section 2.3 since they put it much more concisely than I could: "The magnetic field is always expelled from a superconductor. This is achieved spontaneously by producing currents on the surface of the superconductor. The direction of the currents is such as to create a magnetic field that exactly cancels the applied field in the superconductor. It is this active exclusion of magnetic field – the Meissner effect – that distinguishes a superconductor from a perfect conductor, a material that merely has zero resistance. Thus we can regard zero resistance and zero magnetic field as the two key characteristics of superconductivity." (http://openlearn.open.ac.uk/mod/ouconte ... ection=2.3)

This comes from Lenz's Law, which says that generated current will try to oppose the applied magnetic field (sort of like an electrical version of Le Chantilier's principle if you're into chemistry). The superconductor can do this infinitely well (up to the critical applied field anyway), which leads to the Meissner effect.

There is active research into superconducting transmission lines and also electric machines using liquid nitrogen for cooling. The question isn't whether it's technically feasible to operate it, it's more a question of the cost-benefit of running them. I went to a plenary session in 2008 that hypothesized that electric aircraft of the future could benefit from superconductor-based motors due to the weight advantages.

1 - any article about superconductors that uses "degrees C above absolute zero" instead of Kelvin or negative degrees C is questionable on its face. if you're worried about someone not knowing about the Kelvin scale, why would you think they know what absolute zero is?

2 -

Quote:

Additional phases, including superconductivity, are explicitly quantum in character, relying as they do on the ordering of electric charge carriers.

not all "quantum phases" rely on the ordering of electric charge carriers - this sentence is only accurate if you're only talking about superconductivity. so, either the concept here is inaccurate, or your wording of the sentence is... unfortunate.

in general, this article is not written well. i'm not sure i know (from the above) what a QCP is. your description of the London penetration depth needs some work, particularly in conjunction with your statement that QCP is tied to absolute zero. those are just the factual errors - the writing is confusing in style as well. you name non-intuitive concepts without describing them, and mix them in ways that doesn't appear to convey anything meaningful even to someone who knows what's going on. i know for a fact that this would not fly in a conference of physicist that have intimate knowledge of the subject matter, and it certainly doesn't work in the context of teaching a slice of science to the less-initiated masses. you need to write better, and you need to work more at understanding the gist of the paper you're reviewing and then conveying that gist in an understandable way.